1. Field of the Invention
The present invention relates to a common mode radiation inhibit circuit for a differential transmission line, and electronic equipment. For example, this relates to a common mode radiation inhibit circuit for the differential transmission line corresponding to a measure against EMC (ElectroMagnetic Compatibility) which prevents occurrence of spurious radiation noise in a differential transmission line which transmits a signal at a high speed with a wiring pattern, a cable, or the like on a printed wiring board.
2. Related Art of the Invention
Heretofore, although a single end signal which performs logic oscillation in a supply voltage has been used for transmission of a high speed signal, the cases where Low Voltage Differential Signaling (LVDS) technology is utilized have increased in number from the viewpoint of spurious radiation noise suppression and tolerance over external noise in response to rise of a drive frequency and expansion of bus width which accompany a request of high speed data transfer in recent years. For example, this is used for communication equipment, a digital interface for a PDP, a liquid crystal panel, or the like.
Generally, in LVDS, a differential driver IC is designed so that only differential mode currents in opposite phases may flow between two transmission lines through which a differential signal flows.
FIG. 5 shows a structural diagram of structure of a conventional LVDS interface.
A differential transmission line is constituted of a pair of signal wiring 55 on a printed wiring board 50 and is constituted of a pair of signal wiring 55 in a cable 52, and a printed wiring board portion and a cable portion are connected through a connector 51. In addition, a signal ground (SG) is constituted of a ground plane 56 on a printed wiring board 50 and is constituted of ground wiring 57 in a cable 52, and a printed wiring board portion and a cable portion are connected through a connector 51.
Two lines of signal wiring 55 which are a plus line and a minus line and whose odd symmetric mode impedance is 50 Ω connect a differential driver IC 53 and a differential receiver IC 54, and the signal wiring 55 is terminated by a 100-Ω terminating resistor 62 in an input terminal of the differential receiver IC 54. Since electrical characteristics of the plus line and minus line of the signal wiring 55 are equal, a balanced transmission line is formed, and one signal is transmitted by these two transmission lines in LVDS.
The differential driver IC 53 drives about 3.5 mA of current, and generates a differential signal which generates a potential difference between the plus line and minus line of the signal wiring 55 on the basis of an input signal from an input terminal. The differential receiver IC 54 receives about 350 mV of differential signal generated at both ends of the 100-Ω terminating resistor 62 arranged at receiving ends between the plus line and minus line of the signal wiring 55, and converts this into a CMOS level to output this from an output terminal.
In the LVDS, since signal currents having the same amplitude but being in reverse directions flow in the plus and minus lines of the signal wiring 55, magnetic fields generated according to the currents flowing in respective sides cancel each other, and further, since a signal level is small, occurrence of spurious radiation noise and crosstalk noise is suppressed. In addition, as to external noise, since logic of the signal is not influenced so long as influences in the plus and minus lines of the signal wiring 55 are relatively the same, this is excellent also in noise resistance.
Nevertheless, in transient timing, since it is difficult besides the LVDS to let currents in two differential transmission lines be in opposite phase states exactly, and further, it is theoretically difficult to make characteristics of rise (tr) and fall (tf) of a signal coincide thoroughly, small common mode currents in the same phase flow between two differential transmission lines in the transient timing. In addition, common mode currents occur by a mismatch between differential transmission lines such as a printed wiring board and a cable, and differential impedances of a termination circuit, and a skew between differential transmission lines, and the like.
In FIG. 5, a differential mode current component is matched and terminated by the terminating resistor 62, and a common mode current component returns through stray capacitance, or the like, which the printed wiring board 50 has. Therefore, the common mode current component generated in the differential signal transmission line was a main cause of spurious radiation noise emitted from a LVDS transmission system.
In order to solve this problem, for example, a noise suppression element and a differential transmission circuit which uses this are known (e.g., refer to patent document 1: Japanese Patent Laid-Open No. 2003-258586).
FIG. 6 shows a structural diagram of an LVDS interface which uses the noise suppression element described in the patent document 1. Here, the same reference numerals are used for the same constituents as those in FIG. 5.
According to this structure shown in FIG. 6, common mode currents which flow in two differential transmission lines are inhibited by the noise suppression element which is constituted of a common mode choke coil 59, and transmission noise and spurious radiation noise can be suppressed.
Nevertheless, in the method of using the noise suppression element of the above-described patent document 1, there was a problem that quadratic common mode radiation which is new spurious radiation noise occurrence phenomenon which was not being considered conventionally could not be inhibited.
In FIG. 6, although ground wiring (SG) 57 of the cable 52 and the ground plane (SG) 56 on the printed wiring board 50 are connected through the connector 51, an area size of a signal ground line changes front and back the connector 51, and hence, an amount of a current which flows through a signal ground changes. The quadratic common mode radiation is radiation noise whose source is a common mode voltage generated by change of such a signal ground, and whose radiation source is the signal ground itself.
A subject on inhibiting the quadratic common mode radiation resulting from change of this signal ground is to achieve securement of continuity of a neutral sheet electric potential in a change point of the signal ground which was not considered conventionally in the differential transmission line.
The present invention is devised to solve the subjects mentioned above, and aims at providing a common mode radiation inhibit circuit for a differential transmission line, and electronic equipment which can inhibit quadratic common mode radiation.